Yeah, but, in such case it should be rextest.MatchOffset(
a) in your code, because MatchOffset is accepting index of match, not index of capture.
Index of capture can be used with
GetSubString (argument str_id) as according to docs.
One match can contain several captures. The capture is a part enclosed in parentheses. If you would have several enclosed parts in expression e.g. "(STEAM_)(\\d:\\d:\\d+)", so you'll have several captures in each match.
Anyway, it doesn't change that the result offset is a bugged value. It should point to the start of a string index where match was found.